1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to devices and systems used for marine exploration and, more particularly, to deflectors attached to cables that carry seismic instrumentation.
2. Discussion of the Background
Marine data acquisition and processing generate a profile (image) of a geophysical structure under the seafloor. While this profile does not provide an accurate location of oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of these reservoirs. In order to obtain a high-resolution image of the structure under the seafloor, plural techniques and devices are employed to maintain the data acquisition geometry while scanning the targeted area.
In marine exploration, waves (such as, but not limited to, seismic waves) are directed toward the seabed, and waves reflected back to receivers close to the water surface are used to extract information about the structure and nature of the rock layers underneath the seabed. A vessel usually tows a series of cables connected to a seismic source and/or a streamer that includes the receivers. The vessel tows the cables along a specified path to scan the targeted area. Arranging and maintaining the configuration of these cables (e.g., with minimal drag, the cables remaining at their assigned depth and lateral offset) is desirable. One of the devices employed in order to arrange and maintain the configuration of these cables is a deflector. The deflector is attached to a towing cable and provides a lift force to the cable while towed through the water, to maintain depth and/or lateral separation among the towing cables.
FIG. 1 illustrates a conventional deflector 10 attached to a towing cable 20. The deflector 10 has a flat, rectangular-shaped body 12 and may have ballast bodies 14 and 16 (also known as pods) at ends of the body 12. A top ballast body 14 is lighter than a bottom ballast body 16 to move the gravity center of the deflector 10 below a buoyancy center thereof. The deflector 10 may also have a slot 18 along the body 12 to increase the stalling angle. The length of the deflector may be around 2.5 m. The deflector 10 is attached to the cable 20 around a middle area of the deflector 10.
A problem with the conventional deflector is that its angular stability is low, that is, if it rotates (e.g., due to currents) around the towing position, the deflector does not return to the intended position, and, therefore, instead of providing a lift force, drags down the towing cable. FIGS. 2A and 2B illustrate this problem.
Thus, it is desired to produce a deflector that has better angular stability such that if it rotates around the towing position, it develops a returning torque that returns the deflector to the intended position.